The present invention is related to functional magnetic resonance imaging (fMRI) methods.
Functional magnetic resonance imaging (fMRI) has been used widely in brain imaging studies for the past several years. See e.g., J. W. Belliveau, D. N. Kennedy, R. C. McKinstry, B. R. Buchbinder, R. M. Weisskoff, M. S. Cohen, J. M. Vevea, T. J. Brady and B. R. Rosen, xe2x80x9cFunctional mapping of the human visual cortex by magnetic resonance imaging,xe2x80x9d Science 254, 716-719, 1991; K. K. Kwong, J. W. Belliveau, D. A. Chesler, I. E. Goldberg, R. M. Weisskoff, B. P. Poncelet, D. N. Kennedy, B. E. Hoppel, M. S. Cohen, R. Turner, H.-M. Cheng, T. J. Brady and B. R. Rosen, xe2x80x9cDynamic magnetic resonance imaging of human brain activity during primary sensory stimulation,xe2x80x9d Proc. Natl. Acad. Sci. USA 89, 5675-5679, 1992; P. A. Bandettini, E. C. Wong, R. S. Hinks, R. S. Tikofsky, and J. S. Hyde, xe2x80x9cTime course EPI of human brain function during task activation,xe2x80x9d Magn. Reson. Med. 25, 390-397, 1992; S. Ogawa, D. W. Tank, R. Menon, J. M. Ellerman, S.-G. Kim, H. Merkle, and K. Ugurbil, xe2x80x9cIntrinsic signal changes accompanying sensory stimulation: functional brain mapping with magnetic resonance imaging,xe2x80x9d Proc. Natl. Acad. Sci. USA 89, 5951-5955, 1992; and R. Menon, S. Ogawa, D. W. Tank, and K. Ugurbil, xe2x80x9c4 Tesla gradient recalled echo charateristics of photic stimulation-induced signal changes in the human primary visual cortex,xe2x80x9d Magn. Reson. Med. 30, 380-386, 1993. One of the most often used methods is the gradient-recalled echo-planar imaging (EPI) technique because of its good sensitivity to the blood oxygenation level dependent signal and high speed. See S. Ogawa, R. S. Menon, D. W. Tank, S.-G. Kim, H. Merkle, J. M. Ellerman and K. Ugurbil, xe2x80x9cFunctional brain mapping by blood oxygenation level-dependent contrast magnetic resonance imaging,xe2x80x9d Biophys. J. 64, 803-812, 1993. However, its usage is limited in areas with severe static inhomogeneity induced by susceptibility effect near air/tissue interfaces.
One potential problem in gradient-recalled EPI using a long echo time is the severe signal losses at areas with large static inhomogeneities. These areas include the ventral frontal, medial temporal and inferior temporal regions that experience inhomogeneities induced by the susceptibility effects near air/tissue interfaces. For fMRI studies that use both the gradient-recalled EPI and high field scanners, these signal losses may prevent investigation of the human cognitive processes such as the memory and attention studies. Methods have been developed to recover the signal losses, however, these methods typically involved multiple excitations, thus, compromising the temporal resolution.
Susceptibility artifacts can be manifested primarily in two ways: signal losses and geometric distortions. In general, a long echo time makes an MRI system more prone to signal losses in the presence of an inhomogeneous field because of the intra-voxel dephasing, and a long readout time typically leads to geometric distortions due to the reduced sampling frequency and reduced readout gradient strength. Typically, pronounced susceptibility-related field variation along the slice-selective direction in combination with a long echo time results in severe signal losses, while the inhomogeneity in-plane combined with a long readout time leads to geometric distortions. Thus, signal losses can be caused by the susceptibility-induced gradient along the slice-selective direction. Because of the long echo time typically used in fMRI experiments to maximize the sensitivity toward the signal changes, the signal losses at areas near air/tissue interfaces may be severe. Refined methods to recover these signals may be needed in order to study brain function at these areas.
Several research groups have addressed these sorts of signal losses using various techniques. One such technique is to use a thinner slice thickness to reduce the field change across the slice. See I. R. Young, I. J. Cox, D. J. Bryant, and G. M. Bydder, xe2x80x9cThe benefits of increasing spatial resolution as a means of reducing artifacts due to field inhomogeneities,xe2x80x9d Magn. Reson. Imag. 6, 585-590, 1988. This technique may be relatively easy to implement but it may reduce SNR as well as the spatial coverage per unit time.
Frahm et al. originally proposed to use multiple refocusing gradients to effectively compensate the field inhomogeneities. J. Frahm, K. D. Merboldt, W. Hanicke, xe2x80x9cDirect FLASH MR imaging of magnetic field inhomogeneities by gradient compensation,xe2x80x9d Magn. Reson. Med. 6, 474-480, 1988. This method was later adopted by several other groups and applied more recently in functional MRI. See e.g., R. J. Ordidge, J. M. Gorell, J. C. Deniau, R. A. Knight, J. A. Helpern, xe2x80x9cAssessment of relative brain iron concentrations using T2-weighted and T2*-weighted MRI at 3 Tesla,xe2x80x9d Magn. Reson. Med. 32, 335-341, 1994; R. T. Constable, xe2x80x9cFunctional MR imaging using gradient-echo echo-planar imaging in the presence of large static field inhomogeneities,xe2x80x9d J Magn. Reson. Im. 5, 746-752, 1995; Q. X. Yang, B. J. Dardzinski, S. Li, P. J. Eslinger, M. B. Smith, xe2x80x9cMulti-gradient echo with susceptibility compensation (MGESIC): demonstration of fMRI in the olfactory cortex at 3T,xe2x80x9d Magn. Reson. Med. 37, 331-335, 1997; R. T. Constable, D. D. Spencer, xe2x80x9cComposite image formation in z-shimmed functional MR imaging,xe2x80x9d Magn. Reson. Med. 42, 110-117, 1999; and V. A. Stenger, F. E. Boada, and D. C. Noll, xe2x80x9cGradient compensation method for the reduction of susceptibility artifacts for spiral fMRI data acquisition,xe2x80x9d Proc. ISMRM, p. 538, 1999.
Because the superimposed gradient field across the slice is often not linear, one compensatory gradient is generally not sufficient to compensate the entire slice. Thus, a set of linear gradients is typically needed to compensate the nonlinear field segment-by-segment to achieve satisfactory results. When the number of the linear gradients increase, i.e., increments become finer, the nonlinear field can be better compensated. Despite the effectiveness in recovering signal, the time-consuming nature of such techniques may limit their practical value in routine fMRI experiments. In practice, as many as sixteen repetitions may be needed to sum up to a uniform image. Most of cognitive fMRI experiments cannot be performed this way.
More sophisticated methods were also proposed that showed promise in reducing the number of compensating gradients to a much more tolerable level. The efficiency is much increased by using high-order field compensation. Cho et al. proposed tailored pulse with a quadratic profile that has shown improved tolerance toward field inhomogeneity. Z. H. Cho, and Y. M. Ro, xe2x80x9cReduction of susceptibility artifact in gradient-echo imaging,xe2x80x9d Magn. Reson. Med. 23, 193-196, 1992. Glover et al. also presented a method using high order phase compensation by obtaining a field profile for each subject and incorporating it into the phase profile of the excitation pulse. G. Glover, S. Lai, xe2x80x9cReduction of susceptibility effects in fMRI using tailored RF pulses,xe2x80x9d Proc. ISMRM, p.298, 1998. A similar concept was used in a recent report using a two-shot technique with explicitly matched RF excitation. N. K. Chen, A. M. Wyrwicz, xe2x80x9cRemoval of intravoxel dephasing artifact in gradient-echo images using a field-map based RF refocusing technique,xe2x80x9d Magn. Reson. Med. 42, 807-812, 1999. Another recent method used a two-shot technique combining a quadratic excitation pulse and the compensatory gradient. J. Mao, and A. W. Song, xe2x80x9cIntravoxel rephasing of spins dephased by susceptibility effect for EPI sequences,xe2x80x9d Proc. ISMRM, p.1982, 1999. The resultant phase profile can be used to better match the susceptibility-induced gradients when an appropriate compensatory gradient is used. The two excitations can be implemented back-to-back within one run to allow fMRI experiments to be carried out; however, the effective repetition time is still doubled. Images from the two excitations can then be combined to achieve uniform spatial coverage across the inhomogeneous areas.
The present invention provides systems, methods and computer program products which provide fMRI signal recovery from a single excitation. Such methods, systems and computer program products may be particularly suitable and useful for fMRI studies and applications carried out in or about inhomogeneous areas with high temporal resolution.
In particular embodiments of the present invention, signal recovery in functional magnetic resonance imaging (fMRI) is provided by generating a single excitation pulse and exciting a target region of a subject with the generated excitation pulse. A first image is obtained using a first partial k-space frame of the target region. A compensation pulse is generated and the target region subjected to the compensation pulse. A second, compensated, image is obtained subsequent to the target region being subjected to the compensation pulse using a second partial k-space frame of the target region. The first and second images are combined to form a combined image of the target region. The first and second obtaining steps are carried out sequentially during a single quadratic excitation pulse.
In particular embodiments of the present invention, the single excitation pulse is a matched quadratic excitation pulse. Furthermore, the compensation pulse may be a z-shimming pulse.
In further embodiments of the present invention, the first partial k-space frame has an associated first sampling direction and the second partial k-space frame has an associated second sampling direction. Preferably, the first sampling direction and the second sampling direction are substantially the same direction.
In still additional embodiments of the present invention, the first partial k-space frame and the second partial k-space frame are sampled so that a center of each partial k-space frame is proximate. In such embodiment, the first image may be obtained by completely sampling a first half of the first partial k-space frame and partially sampling a second half of the first partial k-space frame. The second image is then obtained by partially sampling a first half of the second k-space frame and completely sampling a second half of the second k-space frame. Preferably, the sampling of the first half and the second half of the first k-space frame and the sampling of the first half and the second half of the second k-space frame are carried out in substantially the same direction.
In additional embodiments of the present invention, the Echo Time (i.e., the time difference between the excitation pulse and the center of k-space acquisition) (TE) associated with the first partial k-space frame and the TE associated with the second partial k-space frame are asymmetric with reference to the beginning of the respective partial k-space frames and are mirrored about a division between the first and second partial k-space frames.
In still further embodiments of the present invention, alignment indicia are inserted in the first and second images and the alignment indicia are aligned in each of the first and second images when the images are combined. In particular, the alignment indicia may be a centerline of the first k-space frame and a centerline of the second k-space frame.
In yet other embodiments of the present invention, a centerline of the first partial k-space frame is sampled in two sampling directions and a centerline of the second partial-k-space frame is also sampled in two sampling directions.
In additional embodiments of the present invention, the target region of a subject is an in vivo ventral frontal or inferior temporal area of the human brain. Furthermore, the combined image may provide information about the function of the human brain including human memory and attention processes in areas near air/tissue interfaces.
While embodiments of the present invention are described above with reference to methods, as will be appreciated by those of skill in the art, embodiments of the present invention may also be provided as systems and/or computer program products.